1. Field of the Invention
The invention is directed to improvements in the blending of UO.sub.2 and second cohesive powders utilizing a fluidized bed blender and in particular to an improved method and apparatus for converting a hetergeneous mixture of solid UO.sub.2 powders and second cohesive powders into a homogeneous blend.
2. Description of the Prior Art
The blending of particulate solids has been accomplished in the past in a variety of ways. Mechanical mixers of several types, such as tumble mixers, ribbon blenders and high shear mixers have been used. Spouting bed blenders and fluidized bed blenders have also been employed. In the prior art, UO.sub.2 powders have primarily been blended with mechanical tumble-type blenders. This type of blender has not been satisfactory in producing blended batches meeting certain UO.sub.2 powder homogeneity specifications. Failure to meet homogeneity specifications is thought to occur because of stagnant or dead zones within fluidized beds of ingredients being mixed in the blender and segregation problems during discharging. U.S. Pat. No. 4,168,914, issued to Larson et al. and assigned to the same assignee of the present invention, discloses an improved bubbling-bed fluidized bed blender. The Larson et al. blender eliminates the large dead zones encountered in prior art bubbling-bed fluidized bed blenders by providing an apparatus for containing the heterogeneous powders, preferably UO.sub.2 powders, to be blended and comprises a vertically-oriented, slab-shaped mixing vessel having a fluidizing grid disposed at the bottom of the vessel. The fluidizing grid constructed according to the teaching of Larson et al. comprises a linear array of generally downwardly directed pyramidally-shaped hoppers each having walls converging into a conically-shaped opening. A plurality of gas orifices are provided for directing a flow of fluidizing gas upwardly into the bottom of each of the hoppers. Fluidizing gas is supplied to each of the orifices at a velocity sufficient to cause bubbles of fluidizing gas to rise through the mixture of powders and emerge from the powders for thus agitating the powders until a homogeneous blend of powders is achieved. The combination of the linear array of hoppers and the upwardly directed gas orifices eliminates the dead zones encountered with previous bubbling-bed fluidized bed blender designs.
However, of the known prior art apparatus for blending fine and cohesive powders in a fluidized bed blender, none have been effective in meeting product homogeneity specifications in the blending of UO.sub.2 powders and secondary powder mixtures of widely different physical properties. More specifically, and due to their cohesive natures, some second powder mixtures including those which are hydrophobic, hydrophilic or hygroscopic, easily form large agglomerates which produce a poor dispersion. Additionally, certain low density agglomerates will undesirably classify or separate in a fluidized bed blender from the primary UO.sub.2 powder due to stagnant or dead zones that exist at the bottom of the fluidizing bed. Furthermore, at certain gas flow rates utilized in introducing the second powder mixtures an inconsistency in the blending of the powders has been experienced.